Liquid-crystal displays are most important display devices in multimedia societies, and are in extensive use in applications ranging from cell phones to computer monitors, notebook type personal computers, and TVs. Many optical films are used in liquid-crystal displays in order to improve displaying characteristics.
In particular, optical compensation films play a major role in contrast improvement, color tone compensation, etc. in the case where the display is viewed from the front or oblique directions. The optical compensation films which have used hitherto are stretched films of polycarbonates, cyclic polyolefins, or cellulosic resins. However, these films have problems, for example, that a biaxially stretching step is necessary and that it is difficult to conduct the biaxially stretching step so as to attain evenness of retardation. Furthermore, especially in a film having a large area, it is more difficult to regulate the retardation to be imparted by biaxial stretching.
As a technique for eliminating those problems associated with the biaxial stretching, optical compensation layers are being investigated which are formed by coating fluid application (coating) and show an optically compensating function in an unstretched state.
Harris and Cheng of The University of Akron proposed optical compensation layers constituted of a rigid rod-shaped polyimide, polyester, polyamide, poly(amide-imide), or poly(ester-imide) (see, for example, patent documents 1 and 2). These materials have the property of spontaneously undergoing molecular orientation and, hence, are characterized by coming to show retardation through coating fluid application without via a stretching step.
Furthermore, an optical compensation layer formed from a polyimide having improved coating fluid applicability (solubility in solvents) (see, for example, patent document 3), a polarizer having a protective film coated with a diacotic liquid-crystal compound (see, for example, patent document 4), and the like have been proposed.
A stretched film made of a phenylmaleimide/isobutene copolymer has also been proposed (see, for example, patent document 5).
Patent Document 1: U.S. Pat. No. 5,344,916
Patent Document 2: JP-T-10-508048
Patent Document 3: JP-A-2005-070745
Patent Document 4: Japanese Patent No. 2565644
Patent Document 5: JP-A-2004-269842